Preventing corrosion of ferrous metals by ammoniacal solutions of ammonium nitrate



1945- c. K. LAWRENCE ET AL 2,366,796

PREVENTING CORROSION OFFERROUS METALS BY AMMONIACAL SOLUTIONS OFAMMONIUM NITRATE Filed Feb. 28, 1941 7 7 F l ll INVENTOR5 ("HA/H55 ATTOR N EY Patented Jan. 9, 1945 PREVENTING CORROSION OF FERROUS METALS BYAMIMONIACAL SOLUTIONS OF AMMONIUM NITRATE Charles K. Lawrence,Baldwinsville, and Robert F. Engle, Syracuse, N. Y., assignors to TheSolvay ProcessCompany,NewYork, N. Y., a corporation of New YorkApplication February 28, 1941, Serial No. 380,994

11 Claims.

This'invention relates to a process for preventing corrosion of aferrousmetal in contact with aqueous solutions of ammonium nitrate andparticularly of aqueous ammoniacal solutions of ammonium nitrate.contact with the solution. As in the methods For many years past aqueousammoniacal soludescribed above, a disadvantage of this method tions ofammonium nitrate havebeen on the maris the necessity for dissolving inth ammonium ket particularly as a material for use in thepronitrate-ammonia solution a, foreign material. duction of fertilizers.The tank cars commonly While non-ammoniacal aqueous solutions ofavailable for the shipment of these solutions as ammonium nitrate do nothave the high corrowell as the equipment in which they are handledsiveness towards ferrous metals that the amby the fertilizermanufacturers are made of varmbniacal solutions do, nevertheless suchsolutions ious ferrous metals such as iron, steel and special arecorrosive towards these metals. For example, a l y e e e s t s fir, 001-a saturated aqueous ammonium nitrate solution rosive towards the ferrousmetals. For example, was found to corrode a hot rolled, mild steel at ahot rolled, mild steel in contact with a solution the rate of 0.006 inchpenetration per year and containing ammonium nitrate, am-' the presencein such a solution of as little as 0.1% monia and 20% water was found tobe corroded nitric acid increased the rate of corrosion to 0.047 at therate of 0.55 inch penetration per year. A inch penetration per year.Such non-ammoniacal saturated aqueous solution of ammonium nitrate 20ammonium nitrate solutions are frequently encontaining as little as 1%ammonia was found to countered, in commercial practice and. in somecorrode this steel at the rate of 0.57 inch penetracases it is notpossible to employ in such solution P Y tions certain inhibitors whichcan be successfully In view of the highly corrosive nature of these usedin the ammoniacal solutions. For example,

solutions, extensive research has .been carried out in producingammonium nitrate by reaction of to find methods for sufllcientlyreducing the rate ammonia and nitric acid fed into a saturator ofcorrosion of ferrous metals by these solutions vessel containing anaqueous ammonium nitrate $0 t WOuld be p a e t S re, ship and solution,it is frequently desirable to maintain handle them in iron and steelequipment. One a low concentration of nitric acid in the solution. typeof method developed involved the addition It is impracticable todissolve in such a solution to. the solution of certain inhibitors. Forexa thiocyanate as an inhibitor and yet it would ample, as disclosed inU. S. P. 2,215,077, which. be advantageous to have available some meansissued September 17, 1940, to Beekhuis and Mafor reducing the rate ofattack by the acid-amcomber, the addition of a small amount of cermoniumnitrate solution of the ferrous metal used tain sulfur compounds, e.g.,' ammonium thioas a material of construction for the saturator.cyanate, greatly reduced the rate of attack of It is an object of thisinvention to provide a iron and steel by the solution. While theaddimethod for reducing the rate of corrosion of a tion to the solutionof such inhibitors has made ferrous metal by an aqueous am nitrate itpracticable to handle the'solutions in 'apparasolution which may be usedwithout the need for tus made of ferrous metals, even with theseincontaminating the solution by addition thereto of hibitors presentthere is generally some residual foreign substances, or may be used inconjunction small rate of corrosion of the metal by the soluwith the useof chemical agents, such as inhibition. Furthermore, under someconditions of tors, to further reduce the residual slow rate ofproduction, shipment or use of these solutions, corrosion of a ferrousmetalby the inhibited it would be'advantageous if some means weresolution.

available whereby corrosion of ferrous metals by thesolutions could beprevented without having to add a foreign material to the solution, withthe attendant expense of providing the foreign material for addition tothe solut on and contamination of the solution therewith.

It has also been proposed, in U. S. P. 2,135,160, 7-. which issuedNovember 1,' 1938, toHerman A. Beekhuis, Jr., to passify a ferrous metalby treatment with a strongly oxidizing agent such as chromic acid andthen to contact the passified metal with an ammoniacal solution ofammonium nitrate in which a chromate has been dissolved to maintain thepassivity of the ferrous metal in We have discovered that by making thesurfaces of a ferrous metal in contact with an aqueous solution ofammonium nitrate the anode in an electric circuit which includes acathode in contact with the solution and a source of'direct electriccurrent which imposes upon the anode an electric potentialwithinaparticular range, corrosion of that ferrous surface is completelyprevented or at least greatly reduced. The range of voltages w thinwhich the potential imposed on the anode must lie will vary somewhat,depending uponthe specific metal of which the anode is constructed,composition of solution with which it is in contact, etc. below thedecomposition potential Of the solution in contact with the inertelectrode used as a cathode and the ferrous metal of the anode and abovethat at which the flow of current acts to promote corrosion of theferrous metal surfaces of the anode.

In case the container for the ammoniacal ammonium nitrate solution is oflarge size, such as a tank car or storage tank of the size customarilyused for the shipment and storage of these solutions, in which it isimpracticable to employ cathodes having surface areas of the same orderof magnitude as the surface areas of the container which are to beprotected against corrosion by the solution, the application to theelectrodes of voltages below the decomposition potential of the solutionwill not of itself be suificient'to prevent corrosion of an initiallyactive metal. In such cases, however,we have discovered that by firstpassivating the metal surfaces and thereafter making the passivesurfaces the anode in contact with the ammoniacal ammonium nitratesolution, applied voltages below the decomposition potential of thesolution and above that at which the flow of current acts to promotecorrosion of the ferrous metal surfaces of the anode will not onlymaintain the anode surfaces passive against attack by the solution butwill even increase the effectiveness of the passive film on the metalsurfaces.

Any method for passivating a ferrous meta I may be used to effect theinitial passivation of the ferrous metal surfaces. Preferably, the metalsurfaces may be treated with an oxidizing agent such as a solution ofchromic acid, dichromate, potassium permanganate or nitric acid, asdisclosed in U. S. P. 2,135,160. The metal surface may be washed with asolution of a strong oxidiz- In all cases the voltage is ing agent. e.g., 10% KMnO4 5% chromic acid (CrOg) solution in water. Even relativelymilder oxidation treatments are effective in conjunction with theelectrolytic treatment of the containers with the ammoniacal ammoniumnitrate solution in contact with the metal surfaces as herein described.Thus, the oxidizing treatment to passivate the metal may be accomplishedby steaming the metal surface at temperatures of at least 85 C. and thenair-drying the surfaces.

In the case of a tank car in which the interior surface of the containerfor an aqueous ammoniacal solution of ammonium nitrate has beenpassified and then made an anode with a cathode dipping into thesolution in the central part of,the tank, the potential applied to thetank and cathode therein should be at least 1.25

. volts and not above 2.2 volts. Somewhat below 1.25 volts the flow ofelectrical current between the electrodes in the solution acts topromote corrosion of the ferrous metal anode by the solution. Above 2.2volts the current acts to electrolytically decompose the solution,liberating gaseous products which, accumulating in the free space in thecontainer above the liquid surface, present a hazardous condition.Furthermore, decomposition of the material in the solution represents aloss of valuable material. With a voltage, in the range 1.25 to 2.2volts. practically complete protection of the ferrous metal surfaces isobtained without the difflculties. pointed out above, encountered atlower or higher potentials.

asearee In employing the above described means for preventing corrosionof ferrous metals by aqueous ammonium nitrate solutions the cathodeimmersed in the solution may be of any electricallyconducting materialsuitably resistant to attack by the solution. For example, aluminum is asuitably inert metal for the cathode in ammoniacal, acid or in'neutralammonium nitrate solutions.

Despite the relatively low potential applied to the electrodes incontact with the solution, protection of the surfaces of largecontainers for the solution, such as tank cars or storage tanks or thecommercial equipment of plants in which the solutions are produced orused, is obtained without having to. unduly complicate the size andarrangement of the cathode in the container. Protection of the ferrousmetal anode surfaces may be obtained with cathodes of very small surfacearea as compared with the anode surface area and unsymmetricallydisposed with respect to the anode surfaces.

Making the container for the ammonium nitrate solution the anode underthe conditions described above has been found to prevent or suitablyreduce the rate of corrosion of the ferrous metal of which the containeris constructed below the level of liquid in the tank. Corrosion maystart, however, at surfaces above the liquid level and then extenddownwardly to and below the liquid level. This is particularly liable tohappen in the case of tank cars in which upper surfaces may beintermittently wetted with the solution when the car is in transit.Corrosion of the container surfaces above the liquid level which may bewetted by the liquid may be inhibited by having present in the ammoniumnitrate solution an inhibitor.

To counteract the tendency of the surfaces at and above the liquid levelto corrode, we prefer to paint these surfaces with a corrosion-resistantpaint after giving the metal surface an oxidizing passivating treatmentin addition to making the surfaces in contact with the solution an anodeunder the above described conditions. Any paint resistant to attack byan aqueous ammonium nitrate solution may be employed. A chlorinatedrubber paint, marketed under the trade name Tornesit has been foundparticularly good for use in a container for ammoniacal ammonium nitratesolutions. Other chlorinated rubber paints, such as that sold under thetrade name Densol or a coat of red lead primer under a coat of achlorinated rubber paint sold under the trade name Socony White may beused. The entire interior surface of the tank car. may, of course, bepainted in the above manner or the painted area may be confined to astrip immediately above and below the normal liquid level.

This painting of the tank not only prevents corrosion immediately abovethe solution level but reduces the amount of currentwhich must besupplied in anodically protecting themetal surfaces by the process ofour invention.

The simplicity of the apparatus required for by ammonium nitratesolutions in accordance with the process of our invention makes thatprocess peculiarly adaptable for use in connec-' tion with the tank carsin which aqueous ammonium nitrate solutions are shipped. Theaccompanying drawing illustrates diagrammatically an apparatus suitablefor carrying out the process of this invention in such a tank car. 7

With reference to the drawing the numeral I aaeavoe indicates the tankfor containing the solution., As is customary in tank cars for liquids,the tank is provided with a dome 2 through which an unloading pipe 3passes to a sump in the bottom of the tank. This tank is carried by aconventional running gear 4 which also carries a storage battery 5. Thisstorage battery may comprise one or more'2-volt lead storage cells orabout 1.5 volt Edison alkaline batteries for delivery of a directelectric current at a poten-- tial of 2 or 1.5 volts, respectively. Analuminum rod 6 extends through a fitting 9 in dome 2 electricallyinsulating the pipe from the dome and reaches downwardly through theinterior of tank I to near the bottom of the tank. Prefer ably a secondaluminum rod 1 extends for some distance along the bottom of the tankand very close thereto. Rod 1 is in electrical contact with pipe and issupported on electrically insulat- -ing supports 8. While pipe. I is nota necessary feature of the apparatus, it is desirable to insureprotection of the tank when practically empty of solution. The positiveterminal of battery 5 is electrically connected with tank I eitherdirectly, as shown in the drawing, or through the running gear of'thecar. The negative terminal of battery 5 is electrically connected withthe top of rod 6. Thus, when an aqueous ammoniacal solution of ammoniumnitrate is introduced into tank I an electric circuit 'is completedthrough the solution in contact with tank I as anode and rod 6, androd.! when this rod is included in'the apparatus, as cathode. Apotential difference of 2 or 1.5 volts is thus applied to the anode andcathode, depending upon the type of battery used.

In employing the process of this invention for preventing corrosion, anordinary tank car supplied for shipping an ammonium nitrate solution wasprovided with an aluminum cathode consisting of a rod passing through aninsulator in the dome in the car with the lower end of the rod extendingwithin six inches of the bottom of the car. This rod corresponded to rod6 in the apparatus of the drawing. As a source of longer standinhowever, the solution started then painted with Densol.

direct electric current two lead storage batteries connected in parallelwere used. The positive terminals of the batteries were connected withthe dome of the car and the negativev terminals with rod 5.

The interior of the car was first steamed at temperatures above 85 C.and then air was admitted to dry the car while it remained at theseelevated temperatures. The dome was then painted on the inside 'with twocoats of "Tornesit. The car was filled with solution containing ammoniumnitrate, 20% ammonia and 20% water to which had been added 0.1% ammoniumthiocyanate nd 0.5%. AS203 as an arsenite. The car was led until thelevel of liquid rose into the dome of the car.

While the car was being filled, the .current passing between the cartankand aluminum cathode rose to a maximum of about I! amperes. Twenty-fourhours after loading the current had decreased to aboutfi'amperes and itcontinued to drop thereafter'so that the daily average current haddropped to 0.7 ampere on the 12th day. With a surface area of the tankin contact with the solution. of about 850 square feet, this was .equalto about 0.0008 ampere of current per square foot of anode area. At theendof twelve days, the battery was disconnected from the car and fora'period of ten days the tank remained passive and unattacked. by thesolution. On

' ammonium nitrate, 20%

year.

' to corrode the interior surfaces of the tank.

As shown by this example, in utilizing the process of our invention itis not necessary, although in some cases it may be. convenient anddesirable, to continuously maintain an electric potential on thecontainer as an anode to prevent corrosion of the container surfaces.,Contact of the ferrous metal surfaces as anode with the ammoniumnitrate solution under the conditions described above forcarrying out aspecific procedure embodying our invention develops in the passivesurfaces of the ferrous metal a passivity of a nature such that it willpersist for a period of time after the source of electric current isdisconnected from the container and prevent substantial corrosion of thecontainer for some time after the electric current has ceased to flow.Accordingly, our invention contemplates a procedure in which a containerwhich is to be filled and later emptied of the ammonium nitratesolution, as in shipping the solution in a tank car, is made the anodein contact with the passivity which will persist and will itself preventattack of the metal by the solution. The electric current is then cutoff from the container, although the solution is still left in contactwith the surfaces thereof. Or a container for the ammonium nitratesolution may be connectedas anode to a source of electric current todevelop this resistance to attack by the solution and thereafter theelectric potential is applied to thecontainer only intermittently, asneeded to maintain the passivity of the container surface against attackby the solution.

The procedure of the above example may be varied in numerous wayswithout, departing from the scope of our invention. For example, theinterior of the tank car may'be dried out in a current of air, withoutfirst steaming it, and

Under these conditions, after filling the car with the ammoniacalsolution of the above example, the average daily,

current passing at 2 volts potential applied to the tank was 7 amperesfor the first day. By the tenth day the current had fallen to a dailyaverage of about 3 amperes. When the car was un-, loaded on the 16th dayinspection of the surfaces of the tank showed no corrosion.

Hot rolled, mild steel which, as pointed out above, was corroded by asolution containing 60% ammonia and 20% water at the rate of 0.5 inchpenetration per year showed no corrosion when first passivated and thenmade the anode in contact with the solution with a potential of 1 voltapplied'to the anode and a cathode in contactwith'the solution. Theaverage current passing was only 0.00035 ampere per square foot of anodesurface.

In contact with the same solution containing 0.1% ammonium thiocyanate,this steel was corroded at the rate of 0.011 inch penetration per0.00011 ampere per square foot of anode surface.

A saturated aqueous solution of ammonium nitrate containing 0.1% nitricacid corroded] this steel at the rate of 0.047 inch penetration per Whenthe steel was made the anode in contact with the solution with apotential of 1.2 volts applied to the electrodes the rate of corrosiondropped to 0.004 inch penetration per year with an averagecurrentpassing of 0.0039 ampere per square foot of anode surface. Theseapplied potentials of 1 to 1.2 volts were effective to substantiallyprevent corrosion of previously passivated anodes with areas muchsmaller than the interior of tank cars used for shipping ammoniacalsolutions of ammonium nitrate, The applied potentials which areeffective to prevent corrosion of the anode surfaces depend upon thetype and size of the equipment in which surfaces are to be protected bythe process of this invention since the interfacial potential dropbetween anode and solution depends upon the size and arrangement of theanode and cathode. The material used for the cathode may have aninfluence upon the range of suitable potentials applied to theelectrodes. Generally, the larger the apparatus, the higher the appliedpotentials which will be used, although in all cases the appliedpotentials are low, e. g., the 1.25 to 2.2 volts which are suitable fortank cars.

The process of protecting a ferrous metal against corrosion by anaqueous ammonium nitrate solution disclosed and claimed in thisapplication is disclosed and claimed in our copending application SerialNo. 380,995, filed February 28, 1941, for protecting ferrous 'metalsagainst corrosion by aqueous solutions of electrolytes which liberateoxygen at the anode when electrolytically decomposed.

We claim:

1. The process for protecting passivated surfaces of a ferrous metalagainst corrosion by an aqueous ammonium nitrate solution contactedtherewith, said ferrous metal being one subject to corrosion by saidsolution, which comprises making said ferrous metal the anode in anelectric circuit completed through said solution in contact with saidpassivated surfaces of the metal, an inert cathod in said solution and asource of direct current at a voltage which is below that at which saidsolution is electrolytically decomposedin contact with said anode andcathode and is above those at which the flow of current increases therate of corrosion of said surfaces by the solution in contact therewithas compared with the rate of corrosion of the same surfaces in contactwith the solution without the application thereto of an electricalpotential.

2. The process for protecting a ferrous metal against corrosion by anaqueous ammonium nitrate solution contacted therewith, said metal beingone subject to corrosion by said solution, which comprises passiv'atingthe surface of said metal which is to be contacted with said solution bytreating said surfaces with an oxidizin agent, thereafter placing saidsurfaces in contact with said solution and making said surfaces theanode in an electric circuit completed through said solution in contactwith said metal, a cath-' aseavae corrosion by the solution, whichcomprises immersing an inert electrode in the solution in saidcontainer, electrically connecting the negative and positive terminalsof a source of direct electric current to said electrode and to saidferrous metal, respectively, thereby making said passivated ferrousmetal surfaces the anode in an electric circuit completed through saidsolution, and by means of said source of electric current maintainingbetween said ferrous metal and said inert electrode a potentialdifference which is below that at which said solution iselectrolytically decomposed in contact with said ferrous metal surfacesand said inert electrode and is above those at which the flow of currentincreases the rate of corrosion of the ferrous metal surfaces by thesolution in contact therewith as compared with the rate of corrosion ofthe same ode in said solution and a source of direct current at avoltage which is below that at which said solution is electrolyticallydecomposed in contact with said anode and cathode and is above those atwhich the flow of current increases the rate of corrosion ofsaid'surfaces by the solution in contact therewith as compared with therate of corrosion of the same surfaces in contact with of a passivatedferrous metal which is subject to 1 surfaces in contact with thesolution without the application thereto of an electrical potential.

4. The process for protecting the ferrous metal surfaces of a containerfor an aqueous solution of ammonium nitrate against corrosion thereby,

said ferrous metal being onewsubject to corrosion by said solution,which comprises passivating said surfaces by treating it with anoxidizing agent, thereafter contacting said surfaces with said solutionand immersing in the solution an aluminum electrode of small size andunsymmetrically disposed with respect to said surfaces of ferrous metal,electrically connecting the negative and positive terminals ofa sourceof direct electric current to said electrode and to said ferrous metal,respectively, thereby making said ferrous metal surfaces the anode inthe electric circuit completed through said solution, and by means ofsaid source of electric current maintaining between said ferrous metaland said aluminum electrode a potential difference below that at whichsaid solution is electrically decomposed in contact with saidferrousmetal and aluminum electrode and above those at which the flow ofcurrent increases the rate of corrosion of the ferrous metal surfaces bythe solution as compared with the rate of corrosion of the same surfacesin contact with the solution without the application of the potential.

5. The process for protecting a ferrous metal against corrosion by anaqueous ammonium nitrate solution contacted therewith, said metal beingone subject to corrosion by said solution, which comprises subjectingthe surfaces of said metal which are to be contacted by said solution toan oxidizing passivating treatment, coating thus treated surfaces with apaint resistant to attack by said solution, and making the thustreated'and coated ferrous metal the anode in an electric circuitcompleted through said solution in contact with said surfaces, a cathodein said solution and a source of direct current at a voltage which isbelow that at which said solution is electrolytically decomposed incontact with said ferrous metal surfaces and said cathode and is abovethose at which the flow of current increases the rate of corrosion ofsaid'metal by the solution in contact therewith as compared with therate of corrosion ofthe same surfaces in contact with the solutionwithout the appli- I against corrosion thereby, said container havinginterior surfaces of a ferrous metal subject to corrosion by saidsolution which comprises subjecting said interior surfaces. of saidcontainer to an oxidizing passivating .treatment, coating thus treatedsurfaces adjacent to and above the level of liquid to be placed in saidcontainer with a paint resistant to attack by said solution, introducingsaid solution into the thus treated container, immersing an electrode inthe. solu-- tion in said container, electrically connecting the negativeand positive terminals of a source of direct electric current to saidelectrode and to said ferrous metal, respectively, thereby making theferrous metal of the container which is in contact with said solutionthe anode in the electric cirat a potential of 1.25 to 2.2 volts, andcompletcuit completed through said solution, and by means of said sourceof electric current maintaining between said ferrous metal and saidelectrade a potential difference below that at which said solution iselectrically decomposed in contact with said ferrous metal'and saidelectrode and above those at which the flow of current increases therate of corrosion of the ferrous metal surfaces by the solution incontact therewith as compared with the rate of corrosion of the samesurfaces in contact with the solution without the application of thepotential.

7. The process for preventing corrosion of a tank car during shipmenttherein of an aqueous ammoniacal solution of ammonium nitrate, thesurfaces of said tank car exposed to contact with said solution beingcomposed of a ferrous metal subject to corrosion by the solution, whichcom prises passivating said ferrous metal surfaces by treatment with anoxidizing agent, introducing said solution into said tank-car and intocontact with the thus passified surfaces, electrically connecting saidferrous metal surfaces to the positive terminal, and electricallyconnecting an aluminum cathode immersed in the solution in the tank carto the negative terminal of asource of direct current at a voltage of1.25 to. 2.2 volts thereby making said ferrous metal surfaces of saidtank car the anode in a circuit completed through said solution and thecathode and substantially preventing corrosion of the ferrous metalsurfaces by said solution.

8. The process for preventing corrosion of a tank car during shipmenttherein of an aqueous ammoniacal solution of ammonium nitrate, said tankcar having surfaces exposed to contact with said solution which arecomposed of a ferrous metal subject to corrosion by the solution, whichcomprises passivating said surfaces by treatment with an oxidizingagent, introducing said solution into said tank car the surfaces ofwhich have been thus passified, immersing in the solution insaid tankcar an aluminum electrode of small size and unsymmetrically disposedwith ing an electric circuit including said ferrous metal as an anode incontact with said solution and an aluminum cathode immersed in thesolution and electrically connected with the negative terminal of saidsource of direct current and preventing corrosion of the ferrous metalincontact with said solution.

' 10. The process for loading and preparing a tank car for shipmenttherein of an aqueous ammoniacal solution of ammonium nitrate, said tankcar having surfaces of ferrous metal to be exposed to contact with saidsolution, which metal is subject to corrosion by said solution, and saidtank car being provided with an aluminum electrode adapted to beimmersed in the solution, which comprises passivating said surfaces bytreating them with an oxidizing agent, electrically connecting saidelectrode and said ferrous metal of the tank car to the negative andpositive terminals, respectively, of a source of direct electric currentat a potential of 1.25 to 2.2 volts, introducing said solution into saidtank car to complete an electric circuit through the solution betweensaid electrode and the ferrous metal surfaces of the tank car and afterintroduction of said solution into the tank car maintaining saidelectrode and surfaces of the tank car electrically connected to saidsource of current for a substantial period of time long enough to impartto said ferrous metal surfaces'a passivity against corrosion by saidsolution which persists when the metal surfaces of the tank car areelectrically disconnected from said source of electric current.

"11. Thie process for protecting passivated surfaces of a ferrous metalagainst corrosion by anaqueous ammonium nitrate solution contactedtherewith, said ferrous metal being one subject to corrosion by saidsolution, which comprises making said ferrous metal the anode in anelectric circuit completed through said solution in respect to saidsurfaces of ferrous metal, and

contact with said passivated surfaces of the metal, an inert cathode insaid solution and a source of direct current at a voltage which is belowthat at which said solution is electrolytically decomposed in contactwith said anode and cathode-and is above those at which the flow ofcurrent increases the rate of corrosion of said surfaces of ferrousmetal by the solution in contact therewith as compared with the rate, ofcorrosion of the same surfaces in contact with the solution without theapplication thereto of an electrical potential, and incorporating insaid solution an inhibitor of corrosion of said ferrous.

metal by the solution.

CHARLES K. LAWRENCE. ROBERT F. ENGLE.

